A thermoplastic elastomer is generally defined as a polymer or blend of polymers that can be processed and recycled in the same way as a conventional thermoplastic materials, yet has properties and performance similar to that of vulcanized rubber at service temperatures. Blends or alloys of plastic and elastomeric rubber have become increasingly important in the production of high performance thermoplastic elastomers, particularly for the replacement of thermoset rubber in various applications.
Polymer blends which have a combination of both thermoplastic and elastic properties are generally obtained by combining a thermoplastic polymer with an elastomeric composition in a way such that the elastomer is intimately and uniformly dispersed as a discrete particulate phase within a continuous phase of the thermoplastic. Early work with vulcanized compositions is found in U.S. Pat. No. 3,037,954 which discloses static vulcanization as well as the technique of dynamic vulcanization wherein a vulcanizable elastomer is dispersed into a resinous thermoplastic polymer and the elastomer is cured while continuously mixing and shearing the polymer blend. The resulting composition is a microgel dispersion of cured elastomer, such as butyl rubber, chlorinated butyl rubber, polybutadiene or polyisoprene in an uncured matrix of thermoplastic polymer such as polypropylene.
Depending on the ultimate application, such thermoplastic elastomer (TPE) compositions may comprise one or a mixture of thermoplastic materials such as propylene homopolymers and propylene copolymers and like thermoplastics used in combination with one or a mixture of cured or non-cured elastomers such as ethylene/propylene rubber, EPDM rubber, diolefin rubber, butyl rubber or similar elastomers. TPE compositions may also be prepared where the thermoplastic material used is an engineering resin having good high temperature properties, such as a polyamide or a polyester, used in combination with a cured or non-cured elastomer. Examples of such TPE compositions and methods of processing such compositions, including methods of dynamic vulcanization, may be found in U.S. Pat. Nos. 4,130,534, 4,130,535, 4,594,390, 5,021,500, 5,177,147 and 5,290,886, as well as in WO 92/02582.
Particularly preferred elastomeric polymers useful for preparing TPE compositions are halogenated random isoolefin copolymers comprising at least halomethylstyrene derived units. Halogenated elastomeric copolymers of this type (referred to as BIMS polymers) and their method of preparation are disclosed in U.S. Pat. No. 5,162,445. Curable TPE compositions containing these copolymers are described in U.S. Pat. Nos. 5,013,793 and 5,051,477, among others.
TPE compositions are normally prepared by melt mixing or melt processing the thermoplastic and elastomeric components at temperatures in excess of 150° C. and under high shear mixing conditions (shear rate greater than 100 l/sec or sec−1) in order to achieve a fine dispersion of one polymer system within a matrix of the other polymer system. The finer the dispersion, the better are the mechanical properties of the TPE product.
Due to the flow activation and shear thinning characteristic inherent in such BIMS polymers, reductions in viscosity values of these polymers at increased temperatures and shear rates encountered during mixing are much more pronounced than reductions in viscosity of the thermoplastic component with which the BIMS polymer is blended. However, minimization of the viscosity differential between the BIMS and thermoplastic components during mixing and/or processing is essential for the provision of uniform mixing and fine blend morphology that are critical for good blend mechanical properties.